A federal district court impermissibly blurred the line between judge and jury when it refused to allow expert testimony linking benzene exposure with a rare form of leukemia, the U.S. Court of Appeals for the First Circuit held March 22 (Milward v. Acuity Specialty Products Group Inc., 1st Cir., No. 09-2270, 3/22/11).

The trial court “crossed the boundary between gatekeeper and trier of fact” when it focused on the weight of the expert’s opinion rather than the scientific method used in its formation, Chief Judge Sandra L. Lynch said.

An expert’s testimony is reliable where its conclusions flow from scientifically sound methodology, including a so-called weight of the evidence methodology, in which the expert’s opinion is premised on the collective strength of all the evidence instead of the viability of each individual piece, the court held.

Professor Michael D. Green, Wake Forest University School of Law, Winston-Salem, N.C., told BNA April 13 that the decision and its blessing of the weight of the evidence approach was “remarkable” and “goes against the grain” of other courts’ Daubert jurisprudence.

It also means that federal judges will have to roll up their sleeves and start taking a hard look at each piece of evidence relied on by experts, he said.

Trial Court Deems Testimony Unreliable

In October 2004, Brian Milward was diagnosed with acute promyelocytic leukemia (APL). He subsequently sued several chemical companies for negligence, alleging that his cancer was the result of several decades of exposure to their benzene-containing products he came into contact with during his career as a refrigeration technician.

During the “general causation” phase of the trial, Milward’s expert witness offered testimony that, in his opinion, benzene is capable of causing APL in humans.

The witness, Dr. Martyn Smith, “is acknowledged as a leading expert on the study of the toxic effects of chemicals and drugs on the human body, with particular emphasis on the mechanisms by which benzene and its metabolites cause damage to both cells and the human organism as a whole,” the court said.

Additionally, his research—funded by the National Institutes of Health—focuses on the causes of leukemia and lymphoma and has spawned over 215 articles in peer-reviewed journals, the court noted.

Despite his credentials, the district court ruled that Smith’s testimony was inadmissible under Fed. R. Evid. 702, stating that it lacked “sufficient demonstrated scientific reliability.”

Does Testimony Rest on ‘Good Grounds?’

In Daubert, the U.S. Supreme Court “vested in trial judges a gatekeeper function, requiring that they assess proffered expert scientific testimony for reliability before admitting it,” the court said.

But “reliability” has proven to be a moving target, it added, with Daubert urging courts to focus “on principles and methodology, not on the conclusions that they generate,” and later decisions allowing judges to question experts’ conclusions that are too far removed from the underlying data.

In the end, the court returned to the language of Daubert and concluded that, “[s]o long as an expert’s scientific testimony rests upon ‘ “good grounds,” based on what is known,’ … it should be tested by the adversarial process, rather than excluded for fear that jurors will not be able to handle the scientific complexities.”

‘Weight of the Evidence’ Method OK

Here, Smith formed his causation opinion using a weight of the evidence methodology, which the court called “scientifically sound and methodologically reliable.”

Smith considered five separate bodies of evidence, which, taken as a whole, led him to proffer the disputed link between benzene exposure and APL, the court explained.

The evidence relied on by Smith included, among other things, scientific studies on topics such as the relationship between benzene exposure and acute myeloid leukemia (AML—of which APL is a subtype), the type of chromosomal damage known to be caused by benzene and its relationship to APL, and epidemiological studies providing data on the relationship between benzene exposure and various subtypes of AML.

In ruling that Smith’s testimony was inadmissible, the district court questioned his reliance on specific evidence related to several of these topics. It “treated the separate evidentiary components of Dr. Smith’s analysis atomistically,” the court said.

But that view missed the point entirely, the First Circuit said:

In Dr. Smith’s weight of the evidence approach, no body of evidence was itself treated as justifying an inference of causation. Rather, each body of evidence was treated as grounds for the subsidiary conclusion that it would, if combined with other evidence, support a causal inference. The district court erred in reasoning that because no one line of evidence supported a reliable inference of causation, an inference of causation based on the totality of the evidence was unreliable.

Statistically Significant Studies Unnecessary

The defendants also questioned the lack of statistically significant epidemiological studies linking benzene exposure with occurrences of APL.

But, as the court pointed out, “Epidemiological studies are not per se required as a condition of admissibility regardless of context.”

Such a requirement would effectively doom claims involving rare diseases such as APL.

“[T]his is a case in which the few studies that differentiate between AML and APL do not offer conclusive statistically significant evidence either way, in part because the rarity of APL makes it nearly impossible to perform a large enough study,” the court said.

“Dr. Smith estimated that in order to obtain statistically significant results, one would need hundreds of thousands of highly exposed workers, the same number of controls, and millions of dollars in funding,” it added.

Judges Kermit V. Lipez and Jeffrey R. Howard joined the opinion.

Blow to ‘Balkanization’ of Diseases

Professor Green, who is a coauthor of the Reference Guide on Epidemiology in the Federal Judicial Center’s Reference Manual on Scientific Evidence, said that while he thinks the court got it right, judges need to be prepared to get their “fingers dirty” going forward.

Where experts rely on the weight of the evidence as the basis of their opinions, “courts are going to have to look at what that evidence is very carefully,” Green said.

Up until this point, courts have been treating each piece of evidence atomistically, Green said. Now, judges have to say, “let’s put these pieces together and see how it comes out,” he added.

Plaintiffs should benefit from this decision because it helps to counteract the balkanization of diseases—like leukemia—into the smallest, most narrow subcategory possible by defendants, as here, Green said.

The evidence available isn’t going to be nearly as good where you’re dealing with an exceedingly rare disease, such as APL, Green pointed out.

But if experts can rely on the weight of the evidence as a whole, they can look for common pathological mechanisms between related diseases and use those facts to support their own causation analysis, he said.